The present invention relates in general to methods and apparatus for cleaning semiconductor processing equipment, and in particular to remote equipment cleaning where the equipment is cleaned without exposure to the surrounding atmosphere.
In general, manufacturing semiconductor devices involves many processing steps performed under extremely clean conditions using semiconductor processing equipment. At any of the processing steps, the manufactured device may be exposed to a seemingly small amount of contamination which may result in a defect to the device. For example, a contaminant particle as small as 100 Å in diameter can result in a fatal defect in a manufactured device. Thus, reducing contamination during semiconductor processing is directly related to device yield and, in turn, profits.
The problem of contamination is exacerbated by the continual reduction in feature size and also by increasing wafer and die sizes. More specifically, decreasing feature size results in potential defects from ever smaller contaminants. Further, increasing wafer and die sizes subjects a high cost, larger die device to failure due to a single contaminant particle. Alternatively, a large number of smaller die devices on a larger wafer are subject to contamination by a single contaminant particle. Thus, as feature size continues to shrink and wafer and die sizes continue to increase, contamination controls must be advanced to allow economically viable semiconductor device manufacture.
One important source of contaminants is the processing equipment itself. Over time, contaminants form on the processing equipment in the form of both particulates and films. It is possible for the contaminants to migrate to the device resulting in short circuits, open circuits, or other manufacturing defects. In turn, these defects reduce manufacturing yields which increase processing costs and reduce profits. To avoid migration of contaminants formed on the processing equipment to a device, processing equipment must be cleaned periodically. The effectiveness and frequency of cleaning directly impacts processing costs and device yields. As cleaning semiconductor processing equipment directly impacts processing costs and device yields, it is desirable to develop new and advanced cleaning methods.
At present, various methods are used to clean semiconductor processing equipment. In general, the equipment can be cleaned either by opening a processing chamber and manually wiping the chamber (wet clean), or remotely by introduction of cleaning elements to a sealed chamber (dry clean). While manually wiping the chamber is effective, it is time consuming and interferes with normal substrate processing.
Alternatively, the equipment can be cleaned remotely. At present, remote equipment cleaning is accomplished using a remote plasma method. Both methods utilize a cleaning precursor that is often a perflouro-compound (PFC) such as NF3 or CxFy. The PFC is dissociated in a plasma to generate highly reactive radicals, such as atomic fluorine (F).
Remote plasma cleaning is a gentle cleaning technique where a remote energy source is used to create a plasma and reactive radicals outside of the processing chamber. Radicals, such as F, then enter the processing equipment and remove contaminants formed on the processing equipment. More specifically, the radicals react with contaminants formed on the equipment walls to form reactant gases that are suitably discharged from the equipment by an exhaust system. While remote plasma cleaning is an effective method, it exhibits a number of drawbacks.
First, as the fluorine radicals are formed in plasma outside the processing equipment and subsequently introduced into the equipment, they are subject to recombination before they react with contaminants in the processing equipment. More specifically, reactive F radicals recombine to form less reactive F2. These less reactive F2 molecules are not capable of cleaning the processing equipment. In a typical example, more than 90% of F radicals recombine without reacting in the cleaning process. This significant recombination loss occurs during transport from the remote plasma source to the equipment to be cleaned and results in low cleaning efficiency and increased cleaning costs.
In addition, remote plasma cleaning does not involve physical sputtering of residue nor heating of the equipment. Without heating and sputtering, remote plasma cleaning proceeds at a slower rate. The slower clean rate reduces cleaning efficiency and increases process costs.
Further, it is difficult to pinpoint the time at which the cleaning has been completed, i.e., when the last contaminant on the equipment has reacted with a cleaning radical so that it can be discharged from the equipment. The difficulty in detection is due to the reliance of typical detection systems on testing of plasma within the equipment. This difficulty in determining the completion of the cleaning results in both inefficiency and potential errors.
In contrast, in-situ RF plasma cleaning involves application of an RF energy source to create plasma within the processing equipment. More specifically, a precursor gas is pumped into the process chamber and subjected to the RF energy source. The RF energy produces a plasma within the precursor gas. Typically, the precursor gas includes some form of fluorine and the reaction creates F radicals. Similar to remote plasma cleaning, the radicals interact with contaminants formed on the equipment walls to form reactant gases that are suitably discharged from the equipment by an exhaust system. Since, the in-situ plasma clean generates the desired cleaning chemistry inside the process chamber, this approach does not suffer from recombination losses due to radical transport from the remote plasma source to the process chamber.
Unfortunately, while the increased concentration of radicals results in higher cleaning efficiencies and effectiveness, the direct exposure to the plasma also damages processing equipment. This damage results in increased equipment wear from exposing processing equipment to plasmas created from highly reactive gases, leading to equipment failure or downtime.
In addition, as much as 90% of the precursor gas remains unreacted during typical in-situ plasma formation. More specifically, dissociation efficiency can be as low as 10% for CF4 used in in-situ RF plasma cleans. As the precursor gases are typically PFCs, the low dissociation efficiency of precursor gas results in significant levels of PFCs emitted from the cleaning process. It is highly desirable to avoid emission of PFCs as they are a contributor to global warming.
Thus, it can be seen from the discussion above that the art would be improved by advanced methods and apparatus for cleaning semiconductor processing equipment to increase cleaning rate and efficiency while reducing PFCs and wear on the equipment.